The present invention generally relates to the leak testing of Emergency Shut Down valves, more commonly known as ESD valves. ESD valves are in use in many commercial and industrial facilities and, in particular, are used on all or virtually all of the approximately 2,000 off-shore oil and gas platforms around the world to isolate the platform in the event of an emergency. The purpose of the present invention is to provide a means for more easily determining, on-line, whether an individual ESD valve on such a platform or in any other location or application might leak if called upon to close in an emergency or other situation, and thus might fail to perform its intended isolation or other function. The need to verify the integrity of ESD valves has been made more apparent by the Piper Alpha disaster in the British sector of the North Sea, which led to the sinking of a platform and a significant loss of life in the late 1980's.
There are many different types of valves on each off-shore platform, and more than ten such valves are ESD valves. The ESD valves are typically placed in a closed position or condition only during normal or other production shutdowns, as well as during emergencies. The majority of the ESD valves are ball valves, while most of the remainder of the ESD valves are gate valves. On the platform, the ESD valves are usually situated in-line or in series with other valves that are also closed off in emergency shutdown and production shutdown situations. However, the ESD valve is considered to be the valve of last resort, the valve that must perform and must provide isolation should all of the other in-line valves fail to close or fail to fully close or to otherwise leak. Since the other upstream, in-line valves usually do properly close and do not usually leak, and because in an emergency shutdown or production shutdown situation, many of the other, in-line valves close before the ESD valve closes, there is often no differential pressure applied across the ESD valve which could produce a detectable leak. Thus, in emergency or production shutdown situations, commonly used methods for detecting leaks in valves such as high frequency acoustic leak detection cannot be used in most cases to identify a leaky ESD valve, and certainly cannot be used to identify a partially leaky ESD valve. A partially leaky ESD valve is defined herein as an ESD valve in which at least one, but not all, of the internal seals is leaky. Usually the other seals, will prevent a partially leaky ESD valve from leaking, but it is still important to be able to detect a partially leaky ESD valve because such an ESD valve could become a fully leaky ESD valve in the next or some future closure attempt, and it would be wise to repair any such ESD valve at the first available opportunity to prevent a possible leaky situation in the future.
Presently on off-shore platforms, in order to identify leaky or partially leaky ESD valves, platform operators sometime resort to pressurizing the inner cavity of each ESD valve with nitrogen to see if the ESD valve can hold the pressure. This is usually accomplished during costly (up to $300,000 per hour) production shutdowns. It would certainly be better if a method and apparatus could be devised to identify leaky and partially leaky ESD and other valves on-line during production, or during planned and unplanned shutdowns, using equipment and techniques that don't involve costly and time consuming additional steps such as injecting high pressure nitrogen.
The present invention provides a method and apparatus for on-line detection of leaky and partially leaky ESD and other valves during actual production as well as during a shutdown, and without employing additional steps such as the injection of nitrogen gas. The method and apparatus of the present invention can be used on the majority of ESD valves that are installed on off-shore platforms during actual production, enabling for the first time, leaky and partially leaky ESD and other valves to be identified periodically and/or continuously without any loss of production or associated costs. For all ESD valves, including the remainder that can not be identified during production, the present invention can be employed following a shutdown, either a planned or an unplanned shutdown. And as indicated before, no additional steps are required.
The present invention involves the analysis of simultaneous dynamic signals from one pressure transducer located in the cavity of the ESD or other valve and another pressure transducer located upstream or downstream from the valve. On most platforms, appropriate pressure taps are already in place on the ESD valves and on the valve piping. The inventive principles, as hereinafter described, are based on combining an understanding of the design of ESD valves, with an understanding of the physical principles involved in the flow of fluids through pipes and the transients that occur in the cessation of such flow, with an understanding of how sound travels in fluid filled pipes, and finally with an understanding of the processing of dynamic signals.